Lithium-ion battery has extensive application in diverse fields of technology. It is progressively more challenging to enhance the performance of the battery, which involves fine balancing amongst various aspects. Side reactions creep in when new combination is used, for example, sudden redox reaction at specific yet broad range of potential applied. In addition to the costs for manufacture, two main concerns are the life cycle and the stability of the battery. It is obvious that safety is the first and foremost.
Recurrently, flammability and explosion of lithium-ion batteries make the headlines of newspapers and cause concerns. The problem is usually attributable to a poor combination of electrode and electrolyte.
A solution to enhance stability would be to use Lithium Iron Phosphate (LFP) as a material for the positive electrode. Cost and pollution concerns are minimal. The special olivine structure of LFP contributes significantly to the low flammability and explosion risk due to improper handling of the battery such as overcharging, over discharging and/or short circuit.
It is common to use nonaqueous organic solvent in the electrolyte of a lithium ion battery. A mixture of two or more carbonate-based electrolytes are prevalent. The commonly used organic solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), or dimethyl carbonate (DMC). The addition of film former agents is also customary. The common film former agents include vinyl carbonate (VC) and fluoroethylene carbonate (FEC).
All of these organic solvents and film former agents have relatively high volatility and flammability. They are more likely to contribute to an explosion in case of improper handling of a battery.
The use of ionic liquids in the electrolyte of lithium ion battery is being extensively explored because of its low volatility and low-combustion properties. It remains as molten salt that exhibits liquid state below 100 degree Celsius and around room temperature. As such they are called room temperature ionic liquids (RTIL) or room temperature molten salts.
1-alkyl-3-methylimidazolium is the most researched. Taking 1-alkyl-3-methylimidazolium tetrafluoroborate as an example, in which the alkyl chain has a carbon number of 1 to 18. In general, the melting point should increase substantially as the number of carbon chains increases. However, this may not always be the case. The melting point of an ionic liquid with 1-alkyl-3-methylimidazolium cation is substantially affected by the anion. There are many uncertainties.
In summary, the melting point of the ionic liquid depends on multiple factors including the number of carbon in the cation and the anion. For example
1-ethyl-3-methylimidazolium Cl has a melting point of 87° C.,
1-ethyl-3-methylimidazolium PF6 has a melting point of 62° C.,
1-ethyl-3-methylimidazolium BF4 has a melting point of 15 degrees.
1-ethyl-3-methylimidazolium AlCl4 has a melting point of 7 degrees, and
1-ethyl-3-methylimidazolium TFSI melting point −3 degrees,
Another problem would be the size of the molecular group of the ionic liquid. There is a general trend that the larger the molecular group, the higher the viscosity and the lower the conductivity. The addition of carbonate electrolytes to overcome the inherent problem with the ionic liquid brings back the aforementioned concerns on volatility, combustion and possible explosion.
The invention seeks to inhibit or at least to mitigate such shortcomings by providing a stable electrolyte useful in battery for multiple fields.